drm/bufs: s/drm_order/order_base_2/

The version offered by the core is ridiculously optimized and
does the same thing. So use it.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@gmail.com>